Dr. Lakshmana Rao Mantri, Assistant General Manager (Designs), and Satya Narayan Kunwar, Project Manager, AFCONS Infrastructure Ltd. , discuss the design and construction challenges of constructing India’s deepest underground metro ventilation and escape shaft, how the issues were addressed, and the preemptive and mitigation measures taken.
East West Kolkata Metro has bagged national and international accolades by constructing India’s first under water twin rail transportation tunnel including the deepest underground metro station of India – the Howrah Metro station, surpassing the Hauz Khas station and Chawri Bazaar station in Delhi. Kolkata East West Metro Project has also added another feather in its cap by constructing India’s deepest metro ventilation cum egress shaft very close to the east bank of the Hooghly River.
India’s deepest underground metro escape shaft, which is not less than an engineering marvel, is now completed. The excavation depth of shaft is 43.5m, which means it can swallow a 14 storied building inside it!
The shaft was constructed under very challenging hydro-geological conditions and at an equally challenging location. This groundbreaking work has pushed design and construction engineering boundaries and helped in achieving this extraordinary feat.
This article outlines the design and construction challenges and how they have been addressed. It also outlines the spirit of challenging the status quo and shows how use of the very basic engineering principles brought about a turnaround and helped us in carving an engineering marvel. It also highlights the preemptive and mitigation measures that were taken, and which can be replicated to deal with other unique challenges, though they may require completely different solutions based on the specific requirements.
a) Precarious Location:
The location of this deepest shaft is very precarious from construction point of view. The shaft is close to the mighty River Hooghly. Circular Railway track runs abutting to the shaft at west end and the overly busy Strand Road at east end. The north and south ends of the shaft are occupied by two bored tunnels, hardly 3.0 m away from the shaft. The ideal construction sequence would have been to construct the shaft before tunnels are bored for avoiding impact of construction of shaft on tunnels. But due to unavoidable circumstances, both the tunnels were done before shaft construction was taken up (figure 1).
At the west bank of the Hooghly River is the deepest Howrah metro station, whereas at the east bank sits the deepest metro shaft. Moving ahead from Howrah station, tunnels took plunge into the Hooghly River at 36m depth and achieved its deepest point, 39m below ground level, at ventilation shaft. This being deepest point between two stations, the shaft accommodates sump and pumping arrangement for tunnel drainage apart from housing tunnel ventilation and emergency egress system. The vertical alignment of the tunnels below river has been kept sufficiently below the riverbed considering safety and stability of the tunnel under all circumstances (figure 2).
b) Innovative Shaft Construction sequence
After studying various options for construction of ventilation shaft, a very distinctive methodology was adopted. Such methodology has been tried for the first time in India. A square box of 13 m was decided to be constructed with the help of temporary diaphragm walls (1.0m thick and 60m deep) which acted as “retaining structure” for supporting external soil during excavation of the shaft. The shaft was then excavated 1.5m deep at a time (total 24 lifts) in a top-down manner and then a circular lining wall, 500mm thick, was cast inscribing the square diaphragm wall box. This sequence was repeated till excavation reached the final level and all the lining walls were cast and finally base slab was completed. These lining walls acted as strut and provided support to temporary diaphragm walls and hence no strutting was used for completing excavation to the tune of 43.5m (figure 3).
c) Unique Hydrogeology
The Shaft is very close to the East Bank of River Ganges and this area has undergone numerous erratic alluvial subsoil depositions and river course transformation cycles. Apart from that, the city of Kolkata dates back to more than 200 years making geology of this area very erratic and full of surprises due to varied land usage over the time. Top 18m of the soil consisted of wooden logs, boulders, traces of coal, steel pieces, bricks and foundations of the past, now non-existent, buildings and installations. Such variations and hidden surprises posed enormous difficulty in mapping the sub-soil strata and carrying out construction activities, especially in construction of Diaphragm walls and piles (figure 4).
The top 18m of soil is very weak organic soft clay having N value of 5 or less. This layer is underlain by a 17m thick fine silty sand, a water bearing strata, which is connected to Hooghly River and has enormous source of water. Since this layer is followed by a 25m thick stiff clay layer, this water bearing strata is, in fact, a confined aquifer running at high pressure. This confined aquifer was considered a potential threat during excavation of shaft. Further the stiff clay stratum is underlain by a coarse sand layer which too is expected to be connected with river and extends much deeper (nearly 200m deep*). This water bearing sandy strata too is a confined high-pressure aquifer, as this sandy stratum is followed by a stiff clay stratum (nearly 200m deep*). The average pressure of this aquifer is 3 bars to 3.3 bars (based on site instrumentation data) acting at the base of stiff clay layer. This huge pressure in lower confined aquifer was a big threat for deep excavation and feared to result into hydraulic heave during excavation, as the factor of safety against hydraulic heave was calculated to be lower than 1.0 much before reaching final level of excavation.
Challenges Faced for Design and Construction
a) Design Challenges
Construction of India’s deepest Ventilation cum Egress Shaft under challenging geo-hydrological condition was not less than creating an engineering marvel. It was, in fact, a unique kind of challenge having no precedence in India or abroad. This situation demanded unique treatment of the problem and pushed everyone to think beyond boundaries and seek innovative design and construction solutions. Such construction sequence as constructing square D wall box and carrying out top-down construction using circular lining wall inscribing the square box has been adopted for the first time ever and there is no precedence anywhere. The construction sequence uniquely fitted into our requirements and needed no additional equipment or resources. All the available resources at the project were used to construct the shaft. Design of lining wall to function as a strut for deep top-down construction was a unique idea and evaluation of its equivalent strut function remained debated due to absence of a clear-cut guideline matching to this situation. But the lining wall performed excellent strut function and kept diaphragm wall deflection within permissible limit.
Since 500mm thick circular lining wall was inscribing a square box, the thickness of wall was varying, the corners of square box being the thickest. This situation was debated extensively as to whether forces exerted from diaphragm walls onto the lining wall, how the forces from D wall will be transferred to the lining wall, and how the forces, eventually, will be translated into compressive force in the lining wall. Whether benefit of circularity could be derived or not. Various ways of load transfer mechanisms from diaphragm walls onto the lining walls were modelled and studied and reinforcement of lining wall was designed choosing most likely situation and rejecting extreme conditions. Since design and construction sequence was quite new, it was necessary to observe closely the structural behaviors and keep adjusting reinforcement requirements, construction sequences and time cycles from time to time. Observatory methods yielded excellent results and helped completing the construction of shaft. The details pertaining to innovative structural design and observations made during construction require a separate case study.
Mitigating challenge of Hydraulic Heave has also been treated very uniquely where strength of soil has been utilized such that the soil cover has been treated as a 9m thick plug negotiating hydraulic uplift through its weight and frictional resistance between shaft and diaphragm wall. Accepted calculation methods ended assessing FOS equal to 0.38, much below the FOS assessed using shaft-plug-friction approach (equal to 1.11). There was no guideline for assessing FOS based on the concept as stated above. Due to its uniqueness and nontraditional way of treatment, this approach too remained debated. Its successful implementation using extensive pre-emptive and mitigation measures along with extensive instrumentation and monitoring will surely go longways and shape the trend of design and construction strategies under similar situations (figure 5).
b) Proximity to the Riverbank and Railway Track
Deep excavation close to riverbank and very close to a running railway track was very challenging. It was very pertinent to adopt a construction methodology that allows minimal deflection of diaphragm walls and thereby minimal settlement in the track. Construction of circular lining wall in the stages provided excellent rigidity to the system. The maximum deflection observed was 35 mm. The estimated deflection for D wall was 45 mm.
c) Surprises Lay Buried Below
Many surprises lay hidden in the top 15m of soil in the form of wooden logs, boulders, bricks, ropes, coal, and steel plates etc. This indicates that this particular area including the location selected for shaft construction has been a prime business location and has witnessed various modes of activities in the distant past leaving traces of activities lay buried in the ground. During construction of diaphragm walls buried objects as mentioned above made grabbing work very difficult. The accuracy of grabbing was of utmost importance for the construction of 60m deep D wall for accommodating 43.5 m deep shaft. Also adjacent Diaphragm walls needed to be accurately interlocked to avoid water ingress from upper water bearing strata during excavation. There were two instances where diaphragm walls could not be constructed at desired spots due to obstructions encountered during grabbing. As a mitigation measure for deviated diaphragm walls, overlapping D walls were constructed to complete the box. Such mitigation attempts cannot be ensured to meet the requirements perfectly and leaves behind chances of imperfections leading to trouble during future activities such as excavation and lining wall construction. The imperfections due to backup panels created problem of water ingress and was mitigated by doing jet grout piles at strategic locations (figure 6).
d) Lower Confined Aquifer and Hydraulic Heave
As pointed out above under geology, the stiff clay is underlain by a very vast water bearing (coarse sand) stratum deriving its water source from river Hooghly and through ground recharges as this sandy stratum gradually opens up to the ground level quite away from the location of ventilation shaft. Huge water sources available to this water bearing strata makes it a mighty confined aquifer running at a very high pressure of 3 bars or above. Dedicated piezometers were installed to measure the water pressure/water table of lower water bearing strata. Seasonal variation of water table suggests that the water table varies to the tune of 3m, peak observed during monsoon and lowest during summers. This translates into hydraulic uplift pressure at the base of stiff clay layer varying between 3 bars to 3.3 bars respectively.
e) Deep Excavation
Excavation depth was very high (43.5m). The depth of soil cover above lower confined aquifer after final excavation was left only 9.0m. Hence the weight of the soil cover after final excavation against uplift pressure was only 38%. The factor of safety being less than 1 against hydraulic heave was prohibitive of carrying out excavation without implementing some or another mitigation measure.
f) Upper Confined Aquifer and Water Ingress
Upper water bearing stratum was initially considered as a discrete water bearing stratum based on initial soil investigation. But, based on further soil investigations and data obtained from piezometric observations throughout alignment it was confirmed that this water bearing stratum is present throughout the alignment. The soil investigation carried out in river Hooghly indicates that this stratum is opening up in the riverbed and taking recharge from the river. This stratum also gradually comes up to the ground level approx. 10 Kms away from ventilation shaft location and gets recharge during rainy season. This is the reason this layer consisting of fine sand and silt has never depleting water source. Generally, this stratum is 4m to 6m thick throughout the alignment, but at ventilation shaft location it is abnormally high and to the tune of 17m thick (figure 7).
g) 3-Water Tables, One Location
This may sound weird that at one location there may exist as many as 3 distinct water tables. Initially this notion was proposed to the designers and consultants. Our DDC and GC were initially hesitant to accept this idea but water table monitoring data using standpipe piezometers confirmed the existence of 3 distinct water tables. Standpipe piezometers were installed in the lower confined aquifer and upper confined aquifer. Water table of soft organic clay (the topmost soil stratum) was obtained from bore holes during soil investigations. Since Hydraulic Heave phenomena was applicable to our case due to lower confined aquifer only, the water table data recorded for this particular water bearing stratum (-14 mMsl) was used to calculate factor of safety (FOS) against Hydraulic Heave. This out of box idea and verification of idea through instrumentation saved us from considering a very high water table (+5.15 mMSl) for calculation of FOS against Hydraulic Heave. Had this concept not been considered, the FOS would always have been less than 1.0, prohibiting excavation of shaft without resorting to very expensive measures (figure 8).
Initial Design Philosophy of Hydraulic Heave
a) Soil Parameters
Tables 1 and 2 show geological profile and design parameters for various soil strata found at ventilation shaft location. This is based on geological investigations carried out for the project.
b) Hydraulic Heave
Confined aquifers run on pressure and exert pressure on overlying soil stratum. The pressure exerted by aquifer is overcome by the weight of soil. As excavation progresses, the soil cover above aquifer reduces and thereby the weight of soil goes down. The risk of hydraulic heave may occur when the weight of soil cover (stabilising forces) reduces considerably such that it is less than or equal to the pressure exerted by the underlying aquifer (destabilising forces).
This risk was associated with lower aquifer only. The upper water bearing strata was cut-off due to construction of diaphragm walls much below this stratum.
As per Euro code 7 for safety against hydraulic heave, destabilizing pore water pressure at bottom of the column should be less than stabilizing total stress at the bottom of the column.
The recommended F.O.S against hydraulic heave is 1.5 (Euro code 7, Table A.17).
For Vent shaft location, however, the Factor of safety without implementing any mitigation measure is calculated as 0.38. Detailed calculation for this is shown in table 3.
The above calculation shows that the factor of safety is much lower than the recommended FOS and there exists possibility of hydraulic heave due to deep excavation.
Initially, it was thought to implement a dewatering regime during excavation inside the shaft in the lower water bearing strata to reduce the hydraulic pressure at the bottom of overlying stiff clay to avoid the risk of hydraulic heave.
c) Dewatering Wells for Lowering Hydrostatic Pressure
PLAXIS, a finite element software package, was used to carry out the seepage analysis. Transient (time dependent) analysis was carried out for 30-day time period, along with implementation of drains to estimate the water inflow into the excavation (figure 9).
The head was kept -38.5 mMSL and the flow was estimated in the PLAXIS model.
Total flow through wells provided in vent shaft = 8.71 m3/hr. As the Hooghly River is at 50-60m from the Vent Shaft location it was expected that the inflow of water will be continuous. Hence it was proposed to install two pumping wells with submersible pumps in Unit 3b Layer (or lower water bearing stratum) for reducing the hydrostatic pressure at bottom of Unit 3a layer (or stiff clay layer). Pumping wells in this layer shall be activated after the excavation reaches 20m below the ground level. As the excavation progresses beyond 20m, the piezometric head shall be maintained minimum of 1 meter below the excavation level by operating dewatering pimps. Table 4 shows summary of the analysis results.
The PLAXIS analysis along with dewatering regime using two borewells showed achievement of required FOS against Hydraulic Heave, but there were practical difficulties indicating that this methodology may backfire.
- The lower water bearing strata is very vast and connected to river. Actual water flow quantum may be far more than estimated through analysis, and borewells may go out of control before the excavation will reach its final level.
- There was odd experience of such dewatering regime under similar circumstances at Howrah Station. Due to enormity of water associated with lower water bearing strata there was hardly any long term impact on water head due to dewatering and operation of borewells.
- With the progress of excavation, the discharge of borewells was found increasing drastically during excavation at Howrah Station, indicating the borewells may fail here too due to excessive discharge. It was also feared that the water may even seep into shaft from the periphery of borewell due to small left out soil plug at the end of excavation.
Revised Design Philosophy of Hydraulic Heave
a) Plug and Shaft Friction
The stiff clay cover (unit 3a) above lower water bearing strata (unit 3b or confined aquifer) had very low permeability (10-9) and high undrained cohesion of 180 Kpa. Laboratory experiments conducted by some researchers suggests that even a small cohesion requires very high hydraulic gradients to lead to the failure of soil structure. With such a high strength clay cover combined with excavation in a very small plan area of the shaft will not lead to hydraulic failure. The stiff clay cover will function as a plug and the friction between clay plug and shaft walls will resist the uplift pressure exerted by the confined aquifer. This was a revolutionary solution and turning point for design and construction of the shaft. This kind of concept was going to be implemented in the field for the first time for underground construction work.
It is evident that the depth of soil cover was very important for revised design consideration. In order to ascertain the extent of stiff clay stratum at Vent Shaft location, confirmatory soil investigation was done. The details are given in the table 5.
Final Excavation Level of Vent shaft is -38.0 mMSL and Diaphragm toe level is -50.0 mMSL. Based on the boreholes data presented, top of Sand layer/Bottom of clay (as presented in the table above) was considered at -47 mMSL for calculation purpose.
b) Safety Check Calculations
• Adhesion factor α
Average undrained cohesion of clay plug encountered between -38 mMSL and -47 mMSL is around 180 kPa. Various literature is published by different authors correlating Adhesion factor (α) and undrained cohesion (Cu) for bored piles. Based on these published works, it is found that for an average cohesion of 180 kPa, α ranges between 0.35 and 0.6 has been proposed by various authors (table 6).
• Factor of safety on Ultimate shaft friction
Owing to the temporary nature of the work, a safety factor of 1.5 is considered on ultimate shaft friction between the clay and the D-Wall to evaluate safe shaft friction.
Considering the level of piezometric head in lower water bearing stratum (Unit 3b) at -14 mMSL, final excavation level as -38 mMSL and Bottom of clay layer as -47 mMSL, factor of safety against uplift of bottom clay plug is checked and is found to be 1.11. Water pressure acting at bottom of Unit 3a is considered as uplift force, Shaft skin friction between the D-Wall & Clay layer and self-weight of clay are considered as resisting forces against this uplift force. A detailed calculation is shown in figure 10.
The cohesive soils do not suffer liquefaction or erosion of soil continuum as is typical with sandy soils. Soil structure failure for such a high cohesion as evident in the concerned clay layer is highly improbable (We did face opinion difference among designers that the high pressure at the bottom of stiff clay layer may lead to failure of soil structure and the clay plug will ultimately thin down and weaken, the same was ruled out by the majority). Hence theoretically, clay plug was found to be safe and there was remote possibility of hydraulic heave, set aside the possibility of something happening suddenly without sufficient warning. However, to reduce the risk of failure of clay plug due to uncertainties involved in ground variation, errors in geotechnical investigation and variation in adhesion factors, additional preemptive and mitigation measures were necessary. The mitigation measures adopted were very elaborate and designed and planned for taking necessary action during any trigger of associated problems.
a) Pumping Wells Outside the Shaft
To enhance the factor of safety of clay plug and shaft friction four 4 pumping wells outside the shaft were recommended. The pumping wells were planned outside excavation area and outside the shaft such that borewells do not suffer excessive flow when excavation progresses. The vastness of water bearing stratum left very little chance of lowering the water bearing stratum. The borewells were extended into the lower water bearing stratum with an intention to regulate the water head to the design value or lower to the extent possible.
b) Estimation of flow
Flow through each pumping well in Unit 3B (deep) layer is estimated based on equations presented in CIRIA 113 .
Total quantity of water from each pumping well within the shaft area is estimated based on equation mentioned below:
Q is the quantity of flow from a partially penetrated well.
k is the permeability of the layer.
D is the thickness of the confined aquifer (estimated as 18m in current scenario);
h is the head to be lowered to at the well.
RO is radius of influence = 3000*h*√k.
rW is radius of well.
Permeability of the Unit 3b (Deep) layer is expected to be in order of 5 x 10-5 m/sec. To account variability in the ground quantum of flow is estimated for two set of permeability of 5 x 10-5 m/sec and 7.5 x 10-5 m/sec.
In general, initial flow of water from pumping well is higher as compared to steady flow. Expected quantity of flow under steady state condition is 400 to 500 lit/min per well. To account flow of initial stage, pumping capacity should higher than steady state condition. Based on previous dewatering experience on the project, it was suggested to install the pump which are having capacity to pump the water 1000 lit/min.
c) Monitoring of standpipe piezometer head
In addition to dewatering, monitoring of piezometer head below clay plug during excavation was carried out. As per the Monitoring data, levels of piezometric head in lower water bearing stratum (Unit 3b) was lower than -14 mMSL at all the 4 standpipe locations during entire excavation period. This was the design water head considered for FOS calculation. Dewatering borewells outside the shaft were planned to be switched on when the water head in the piezometers exceeded -14 mMSL. However, the water table never exceeded -14mMSL during excavation of the shaft. Also the excavation was planned such that it finishes before onset of monsoon which could cause water table to rise depending upon intensity of monsoon.
d) Monitoring of Heave
During excavation heave monitoring was necessary and very vital. The excavation was planned to be carried out in the lifts of 1.5m. During each lift of excavation the soil was expected to relax resulting into elastic rebound. This elastic rebound could be misleading and create a notion of hydraulic heave during various stages of excavation (table 7).
To eliminate the chances of confusion, the elastic heave during each stage of excavation was estimated and used for comparing the actual heave recorded. The “linier elastic continuum approach” has been used for estimation of heave due to removal of over burden soil at various stages of excavation.
In a PLAXIS analysis the rebound due to excavation showed an uplift of 45mm which is quite close to the above calculation. The comparison of box was presented in graph 1.
An FE analysis was carried out using GEOSTUDIO. SEEP/W module was used to carry out Steady State Analysis and Transient Analysis to assess hydraulic condition at the excavation base. The water inflow output at the base of excavation was negligeable. SIGMA/W module of GEOSTUDIO was used to carry out Load – Deformation analysis to assess upward movement of soil mass at excavation level. The upward movement value achieved was 125mm which was primarily attributable to rebound of soil on account of stress relief due to unloading of soil post excavation. The difference in upward movement values between PLAXIS analysis and the analysis using GEOSTUDIO was due to the values of stiffness modulus used for modeling in PLAXIS (Eur) and GEOSTUDIO (E50). PLAXIS uses Eur value which is 3 times the E50 value, and when appropriate adjustments are made the elastic rebound values matched well. Hence all three methods of estimating elastic rebound were consistent and closely matching (figure 11).
e) Monitoring of Soil Parameters
The strength of soil which was adopted as 180 Kpa (undrained shear strength, Cu) was key in assessing FOS against hydraulic heave. The success of this concept was largely dependent on consistency of the soil strength. In order to ascertain the soil strength and to keep watch of its consistency, following measures were taken.
- After each stage of excavation in situ strength of soil was measured with the help of pocket penetrometer. The readings were taken at multiple locations and averaged out to find Cu value. The soil strength was found quite consistent and most of the time exceeded the assumed value (figure 12).
- Also, the soil adhered to the shaft wall was poked manually to see the quality of adhesion. We found that considerable effort was required to dislodge the adhered soil from the shaft wall. This indicated that the shaft and clay cover friction will be adequately achieved. Such field techniques enhanced confidence for carrying out further excavation.
- Moisture content test of the soil was also tested and monitored after each stage of excavation using rapid moisture meter and in the lab. It was assumed that the moisture content of stiff clay will remain the same or reduce with the progress of excavation. The trace of excess moisture compared to the moisture content of the soil from previous stages of excavation would indicate water being forced into the soil from lower water bearing stratum. The moisture content found was between 20% to 22% which represents a near dry stiff clay soil.
CCTV cameras were installed at strategic locations to keep constant watch on excavation activities. These camaras were also connected with mobiles phones of key persons through Camera APP to see and review the situation inside the shaft at any instant. This arrangement gave a good handle on watching and monitoring construction activities inside the shaft.
g) Other Observations
Some other observations were also carried out to keep complete track and capture any sign of distress at the earliest. They may be summarized as follows.
- Photographic observation of interface between D-wall and soil layer after excavation for each lining wall from 20th lift onwards. The photographic observations provided excellent record of soil wall interface conditions. These photographic observations were minutely examined by the experts and consultants including TTAJV team.
- Videography of shaft after excavation was also kept for records. This was helpful for reviewing previous stages of excavation with current stage of excavation in case of requirement.
- Minute examine of D-wall face like any seepage, its locations, its directions of flow. It was also necessary to see if there was any sign of seepage at the soil and wall interface.
- Water content measurement report at site by Rapid Moisture Meter. Also, moisture content was measured in the lab using samples collected from excavation.
- UDS collection for strength test and water content measurement at laboratory by IIEST, Shibpur.
- DDC / GC / IIEST-Shibpur / TTAJV used to visit jointly to assess the physical condition at each excavation stage.
Above preemptive measures were put in place to monitor and regulate the excavation work and keep an eye on the triggers of hydraulic heave. It may also be noted that this phenomenon was not expected to happen overnight and will provide sufficient warning for carrying out mitigation measure to prevent or minimize the impact of hydraulic heave. Following measures were put in place as part of contingency measures that could be run as per need in case of any contingent situation demanding drastic action.
a) Maintaining Stock of Soil Near Shaft
Approx. 500 m3 of soil was stockpiled near shaft location. This soil was secured for backfilling the shaft immediately upon notice of any trigger of hydraulic heave. A platform was fabricated suitably for placing of excavator such that the excavator could quickly push the stored soil into the shaft within a short period of time. A couple of mockups were carried out to make sure that in case of requirement backfilling of the shaft with stored soil can be done without any problem.
It is interesting to note that the excavation of the shaft came to a halt due to the nationwide lockdown imposed during onset of pandemic. This stored soil was then put into the shaft during lockdown period to ensure enhanced FOS of soil cover and eliminate any surprise. The soil was again removed when the lockdown was relaxed and construction work started moving.
b) Storage of Water at Shaft Location
Approx. 4.5 Lakh liter water was stored at the site during excavation period of the shaft. This water was stored in multiple water tanks along with multiple pumps. Appropriate pipeline networks were arranged such that water from all the tanks could be pumped into the shaft simultaneously in case of visible or measured trigger of hydraulic heave. The backfilling with soil and water could be done simultaneously in case of such demand. There was a daily protocol to check the functioning of all the pumps and pipelines so that pumping failures do not occur in case of requirement.
c) Pumping Arrangement from Hooghly River
A pumping arrangement was established from Hooghly River with the help of government authorities. This arrangement was able to pump water from river to the tune of ---- Liters/hour into the shaft. This arrangement was made to address very critical situation requiring drastic action to fill the shaft quickly. In case of distress taking place rapidly, high quantum pumping would be the only solution to bring situation under control in minimum time (figure 13).
If backfilling of soil, pumping of stored water at shaft location and pumping of water from river Hooghly run simultaneously, the shaft could be filled to the desired level within 4 hours from its deepest level of excavation.
Finally, the Engineering Marvel Created Despite COVID-19 Pandemic
The problem of Hydraulic Heave aggravates with prolongation of construction time. This implies that once the work enters critical zone, it should be completed in bare minimum time. Construction of Shaft entered its criticality when the first Lockdown was imposed due to COVID-19. The team developed strong protocol based on guidelines developed by HO to tackle the pandemic and created a strong bubble for Vent Shaft workers. The workers were brought to the site from their homes on dedicated buses and cars, quarantined for 7 days and RTPCR tested. The negative tested workers were allowed for Vent Shaft construction work. The work area being very tight, it was difficult to maintain social distancing and hence it was pertinent to isolate workers from outside contacts. Any spread of virus at Vent Shaft location would have brought the work to a halt, which, in turn, would have increased the risk of occurrence of Hydraulic Heave. With extensive effort and implementation of tight protocol at site, COVID-19 cases could be completely prevented and the construction of Vent Shaft completed successfully. The completion of Vent Shaft was extensively covered by media and appreciated by the engineering fraternity.
Design and construction of India’s Deepest Ventilation cum Evacuation shaft came with unique challenges. Several options for design and construction were worked out but were discarded on some or another grounds. The finally chosen design and construction methodology was fitting into our requirement but still was quite challenging due to its uniqueness and lack of precedence requiring careful analysis and implementation. With this success story it can be concluded that any challenge can be negotiated if we venture to challenge the status quo, follow the basics of engineering and exercise utmost care and precaution. The team left no stone unturned to make it a success with the strong support from our experts and seniors at Head Office.
About the Author:
He has received the Industry Expert National Award 2020 from IRDP Group of Journals; Sir M V Award from Smart Infra EST, Hyderabad in 2020; Sir M V Awards from Karnataka State Engineering Association for providing innovative solutions for landfills in 2014; and Achievement awards from NHAI in 2018 for providing solutions in sustainable Infrastructure in N-E Region.
He is a member of the Indian Geotechnical Society (IGS), Deep Foundation Institute (DFI), Indian Road Congress (IRC) and International Society of Soil Mechanics and Ground Engineering (MISSGE)